Abstract

This SRFI specifies the procedure get-environment-variable, which gets
the value of the specified environment variable, and the procedure
get-environment-variables, which gets an association list of all
environment variables.

Rationale

Most operating systems provide a mechanism for passing auxiliary
parameters implicitly to child processes. Usually, this mechanism is
called "the environment", and is conceptually a map from string names
to string values. The string names are called environment
variables.

Some applications rely on environment variables to modify their
behavior according to local settings. Also, various established
protocols rely on environment variables as a form of interprocess
communication. For example, most implementations of the common
gateway interface (CGI) use environment variables to pass
Meta-Variables from the server to the script [1].
Environment variables are also required
by SRFI 96: SLIB Prerequisites.
Providing a means to access environment variables is therefore
indispensable for writing practical programs in Scheme.

Most widely-used Scheme implementations provide a function for
getting the value of a specified environment variable. The name for
this function is usually getenv, but varies (see
below). Some implementations also provide a way to get all the
environment variables, but others do not.

This SRFI specifies a uniform interface for accessing environment
variables. That should make it easier to write portable programs
that need access to their environment.
For example, a CGI program may portably obtain the values of the
Meta-Variables "QUERY_STRING", "CONTENT_LENGTH" and
"REQUEST_METHOD" as in the following examples:

Specification

Returns the value of
the named environment variable as a string, or #f if the named
environment variable is not found. The name argument is expected to be
a string.
get-environment-variable may use
locale-setting information to encode the name and decode the value of
the environment variable.
If get-environment-variable can't decode the value, get-environment-variable may raise an exception.

scsh

SCM

Issues

get-environment-variable is expected to return a "Scheme string".
Unfortunately, many current platforms, including POSIX-like ones, do
not require environment variables to be interpretable as sequences of
characters. In particular, environment variables can be used to name
files, and filenames on the system can amount to NULL-terminated byte
vectors, which, if the Scheme program were to collect uninterpreted
and pass to, say, the open call, would work just fine, but which might
not represent a string in the particular encoding the program expects.
While in principle it may be desirable to provide a mechanism for
accessing environment variables raw, this SRFI specifies a "string"
return type because that best represents the consensus of existing
implementations, and captures the semantically desirable behavior in
the common case that the byte sequence is interpretable as a string.

Acknowledgements

Thanks to Shiro Kawai, Alexey Radul, jmuk, Kokosabu, leque and all the members of the #Lisp_Scheme IRC channel on Freenode.

Copyright

Copyright (C) Taro Minowa(Higepon) (2008). All Rights Reserved.

Permission is hereby granted, free of charge, to any person obtaining a
copy of this software and associated documentation files (the
"Software"), to deal in the Software without restriction, including
without limitation the rights to use, copy, modify, merge, publish,
distribute, sublicense, and/or sell copies of the Software, and to
permit persons to whom the Software is furnished to do so, subject to
the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

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